3: （This step is an optional step, no difference to do it or not.）
Use OpenIV and extract extratitleupdatedata.meta from: \Grand Theft Auto V\update\update.rpf\common\data\extratitleupdatedata.meta
open it with notepad, and add theses new lines:

dlc_w221s500:/update:/dlc_patch/w221s500/

Save it, and replace the original extratitleupdatedata.meta with this one using OpenIV.

4: To spawn this car, you need use a trainer with "spawn by name" function to spawn the vehicle type in: "w221s500"